1. Field of the Invention
The present invention relates to a developing apparatus which is used in electrophotographic type copy machines and printers. More particularly, the present invention relates to a developing apparatus which easily removes developer remaining on a developer carrier after development of an image. Removal of developer remaining on the development carrier is accomplished by making an elastic member for eliminating charge better contact developer on the developer carrier. The present invention also relates to an image forming apparatus using such a developing apparatus, and a development method carried out by the developing apparatus.
2. Description of the Prior Art
A developing apparatus has been used wherein a thin layer of developer is formed on a developer carrier and latent images are developed with the thin layer of developer. In this conventional developing apparatus, excess toner remains on a developer carrier even after development (i.e., after developer is imparted onto an image retaining carrier). Preferably, the excess toner is once removed from the developer carrier. Then, following the removal of the excess toner, a uniform, thin layer of developer is formed on the developer carrier. However, defective images including uneven density, vertical stripes or the like appear unless a thick uniform layer of developer is constantly formed on the developer carrier during the development operation.
An example of typical structure of such a conventional developing apparatus is shown in FIG. 11. This developing apparatus 100 has a rotating development sleeve 104 contacting a photosensitive drum 40. A supply roller 109 supplies toner to the development sleeve 104. A stirring blade 110 stirs toner present in a developer storage tank 102. A regulating blade 108 and a charge-eliminating sheet 106 are made to press against a periphery of the development sleeve 104.
The regulating blade 108 causes an even, thin layer of toner to be formed on the development sleeve 104 and also negatively charges the toner. The charge-eliminating sheet 106 removes toner remaining on the development sleeve 104 after image development by eliminating charge on the remaining toner. The charge-eliminating sheet 106 is flexible, resists the adhesion of melted toner, and resists abrasion. An electrically conductive member which has some electrical resistivity is used for the charge-eliminating sheet 106. Some electrical resistivity is required so that overcurrent will not flow to the charge-eliminating sheet 106 when toner on the development sleeve 104 runs out and the charge-eliminating sheet 106 directly contacts with the development sleeve 104. More specifically, a conductive fluorocarbon resin sheet or the like is used for the charge-eliminating sheet.
Further, the developing apparatus 100 has an electric power unit 112 which applies both voltage for eliminating charge and bias voltage for developing an image. As shown in FIG. 12, voltage is applied to the charge-eliminating sheet 106 from its end.
The developing apparatus 100 operates as follows. First, toner present in the developer storage tank 102 is supplied to the development sleeve 104 by rotation of the supply roller 109. Then, toner is made into a uniform, thin layer by the regulating member 108 and carried onto the development sleeve 104. Next, the uniform, thin layer of toner is supplied to electrostatic latent images on the photosensitive drum 40 and the images are developed. Toner not used for development (i.e., excess toner remaining on the development sleeve 104 after development of the latent images) is transported by rotation of the development sleeve 104 to come into contact with the charge-eliminating sheet 106. Voltage as described above is applied to the charge-eliminating sheet 106. The polarity of the voltage is opposite the polarity of charged toner. Therefore, the charge on the toner is eliminated. As a result, the charge-eliminated toner can be removed from the development sleeve 104. Subsequently, a new, thin layer of toner is formed on the development sleeve 104.
However, the aforementioned conventional developing apparatus 100 has a problem in that a charge-eliminating voltage cannot be applied uniformly in a longitudinal direction to a toner layer remaining on the development sleeve 104. As shown in FIG. 12, voltage is applied from one of the longitudinal ends of the charge-eliminating sheet 106. Because the charge-eliminating sheet 106 has some electrical resistivity, the potential drop becomes larger as the distance from the point at which a voltage is applied (voltage-applied point) increases. Therefore, the potential of the charge-eliminating sheet 106 is not uniform. Where the voltage drop is large (right side in FIG. 12) the charge-eliminating sheet has a poor charge-eliminating effect.
Moreover, a charge-eliminating voltage cannot be applied to the entire thickness of the toner layer remaining on the development sleeve 104 after development. The developing apparatus 100 can apply a charge-eliminating voltage only to a portion of the toner layer remaining on the development sleeve 104. Since the surface of the charge-eliminating sheet 106 is smooth, the charge-eliminating sheet 106 contacts only the surface portion of the toner layer. Therefore, other than at its surface, the charge on the toner layer remaining on the development sleeve 104 after development is not sufficiently eliminated.
Due to the above-mentioned conditions, charged toner which was not eliminated can remain on the development sleeve 104 as a new thin layer of toner is formed over the remaining toner. The increased amount of toner adhering onto the development sleeve 104 causes defective images containing fog, uneven density, zebra patterns, or other noise. Further, toner which was not eliminated accumulates on the development sleeve 104 during a long period of continuous printing. Consequently, a thin film of resin including a toner component, (filming) is created. Filming causes image defects such as fog. This effect is most noticeable at a zone far from the voltage-applied point where there is little charge-eliminating effect. Further, this effect is very noticeable in an initial printing period when the amount of charged toner is large and the toner is in a low-humidity condition.
It is generally known that the charge-eliminating effect increases when a charge-eliminating sheet more firmly presses against a development sleeve. Accordingly, the above mentioned problems may be mitigated using this technique. However, if the charge-eliminating sheet 106 presses to firmly against the development sleeve 104, other undesirable conditions are created such as irregular driving of the development roller 104, requiring a large-sized driving device for the development sleeve 104. Therefore, it may not be feasible to increase the firmness with which the charge-eliminating sheet 106 presses against the development sleeve 104.